Abstract
Since their development in the 1930s, Schiff-base complexes have played an important role in the field of coordination chemistry. Here, we report the synthetic, spectral, structural, magnetic and electrochemical studies of two new pentacoordinated neutral chloro-iron(III) complexes (3,5) supported by dianionic [N2O2]2− tetradentate Schiff-base ligands unsymmetrically substituted by either a pair of acceptor (F and NO2) or donor (ferrocenyl and OCH3) groups. The electron-withdrawing Schiff-base proligand 2 and the complexes 3 and 5 were prepared in good yields (79–86%). Complex 3 was readily obtained upon reaction of 2 with anhydrous iron chloride under basic conditions, while the bimetallic derivative 5 was synthesized by condensation of the free amino group of the ferrocenyl-containing O,N,N-tridentate half-unit 4 with 5-methoxysalicylaldehyde in the presence of FeCl3. The three new compounds were characterized by elemental analysis, FT-IR, UV–Vis, mass spectrometry and in the case of 2 by multinuclear NMR spectroscopy. The crystal structures of 3 and 5 revealed that in the two five-coordinate monomers, the iron atom showed distorted square-pyramidal geometry, with the N and O atoms of the Schiff-base ligand occupying the basal sites and the chlorine atom at the apex of the pyramid. Magnetic measurements showed a high-spin configuration (S = 5/2) for the Fe(III) ion in 3 and 5. Reduction associated with the Fe(III)/Fe(II) redox couple occurred at −0.464 and −0.764 V vs. Ag/Ag+, and oxidation taking place at the Schiff-base ligand was observed at 1.300 and 0.942 V vs. Ag/Ag+ for 3 and 5, respectively. A high-electronic delocalization of the Schiff-base ligand substituted by fluoro and nitro groups stabilizes the Fe(II) oxidation state and shifts the redox potential anodically.
Highlights
Since their first description by Hugo Schiff in 1864 [1], the so-called Schiff-base compounds have become, in their deprotonated form, one of the most widely used acyclic and macrocyclicInorganics 2018, 6, 5; doi:10.3390/inorganics6010005 www.mdpi.com/journal/inorganicsInorganics 2018, 6, 5 polydentate ligands in coordination chemistry [2,3,4,5]
Schiff-bases that have been characterized by an imine (>C=N–) or azomethine (–HC=N–) group are synthesized by straightforward condensation reactions of ketone or aldehyde with primary amines
N,N-bis(salicylidene)ethylenediamine H2 -salen, and N,N-bis(salicylidene)-o-phenylenediamine H2 -salphen, both reported for the first time by Pfeiffer and coworkers 85 years ago [6], probably represent the archetypical symmetric N2 O2 tetradentate
Summary
Since their first description by Hugo Schiff in 1864 [1], the so-called Schiff-base compounds have become, in their deprotonated form, one of the most widely used acyclic and macrocyclic. Karuppasamy et al reported a series of eight chloro-iron(III) complexes with salen ligands symmetrically substituted with donor or acceptor groups prepared by a solvothermal one-pot method [28]. We successfully prepared a series of robust neutral [M(salphen)]-type complexes (M = Ni, Cu) supported by dianionic [N2 O2 ]2− tetradentate Schiff-base ligands (LD , LA ) substituted by either a pair of donor or acceptor groups [29] Encouraged by these results and continuing our efforts in this field we have extended the scope to pentacoordinated chloro-iron(III) complexes of the type [Fe(LD /LA )Cl] as a preliminary study towards the elaboration of more complex coordination compounds.
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